THE STRUCTURE OF THE UNIVERSE

Cosmic Objects and Their Distances

The universe is populated with objects that are in motion. Take Earth, for example. It orbits the Sun, but it is also moving through space as part of the solar system. The solar system is in motion around the center of the Milky Way Galaxy. The galaxy itself is moving through space along with other galaxies as part of a collection called the Local Group. In some places, galaxies orbit each other within clusters and associations, but all of those galaxies are also moving through space as part of the expansion of the universe that began some 13.8 billion years ago. The universe is also huge. It began as a small point and is now measured at about 93 billion light-years across!

All the matter the universe contains (both baryonic and dark matter) is distributed in a lacy network that stretches across those billions of light-years of space. In between the strands of the network are cosmic voids filled with dark matter. Finally, everything is affected by dark energy as it speeds up the expansion of the universe.

Climbing the Cosmic Distance Ladder

Distances are important in the cosmos. Obviously people explore the closest objects such as the planets and nearby stars and nebulae more easily than the distant stars and galaxies. The farther out we look, the further back in time we see, and so distance becomes an exercise in studying the history and evolution of the cosmos and the objects it contains.

Earth and the Sun are 1 astronomical unit (AU) apart. That’s 150 million kilometers. At the AU level, our exploration is limited to the solar system, which is pretty small in the cosmic order. More distant objects are referred to as lying light-years away. A light-year is the distance light travels in a year at 300,000 kilometers per second. The nearest stars are about 4.2 light-years from the Sun. One of the nearest starbirth regions is 1,500 light-years away. Astronomers also use the term parsec, which is equivalent to 3.26 light-years. Multiples of thousands of parsecs are called kiloparsecs. Measurements in these units get us out to the objects in the Milky Way. So, for example, the center of our galaxy is about 26,000 light-years away, which can also be expressed as 8,000 parsecs, or 8 kiloparsecs.

Beyond Our Galaxy

Outside the Milky Way, astronomers deal in huge distances—measured in millions or billions of parsecs, called megaparsecs and gigaparsecs, respectively. These are often referred to as cosmological distances to bodies such as galaxies. Astronomers can measure these distances using the standard candles mentioned above. Another method, called the Tully-Fisher relation, compares the intrinsic luminosity (that is, how bright it is) of a spiral galaxy to the orbital motion of stars around the center of the galaxy. To get this “rotation velocity,” astronomers study their light through a spectroscope—an instrument that splits light into its component wavelengths. The spectra show how fast the stars are moving as the galaxy rotates.

Sometimes changes in surface brightness (that is, the brightness across an extended object like a galaxy or a nebula) can also be used to determine its distance. This surface brightness method works well with objects more than 100 megaparsecs away.

How the Universe Got to Be That Way

Astronomers detect and study a universe filled with light-emitting objects. How did everything get to be the way it is? A few hundred thousand years after the Big Bang, the infant universe was filled with matter distributed across expanding space. At that time, the universe was much smaller than it is now because it hadn’t expanded very far. In some places, the density of the matter was a tiny, tiny percent higher than in neighboring regions. In those areas, the expansion of the universe was a little slower, allowing the areas of higher density to grow and become more dense. The first stars were formed in those areas of “over-density,” and the regions where they lived and died were the seedbeds of the first galaxies.

From those tiny density variations in the early universe, the largest-scale structures in the universe—the galaxies and galaxy clusters—grew and clumped together under the force of their mutual gravitational attractions. These clusters and superclusters are arrayed in walls, sheets, and filaments of matter that seem to be draped around vast, empty-looking voids. Today people are interested in the role that both dark matter and dark energy played in the creation and ongoing evolution of the stars, galaxies, galaxy clusters, and superclusters that make up this cosmic web of existence.

The fact that so many superclusters exist proves that matter in the universe is not evenly distributed and that it hasn’t been since these clusters first began forming. Today, these structures stretch for hundreds of millions or even billions of light-years, and yet they define only a small percentage of the universe that we can detect.

From the Small to the Large

Matter in the universe is clustered in a hierarchy. Here’s a list of cosmic objects, ranging from small to large.

• Planets
• Stars
• Galaxies
• Groups of galaxies
• Galaxy clusters
• Superclusters (clusters of galaxy clusters)
• Voids between clusters and superclusters
• Filaments of galaxies outlining the voids